Embodiments of the present inventive concepts relates to an apparatus and method of stretch blow moulding, in particular, for stretch blow moulding a plastic preform to form a container. The plastic preform is expanded in a blow mould of a stretch blow moulding machine to form the container. The plastic preform is stretched with the aid of a stretching rod element which can be moved axially into the blow mould in a stretching direction.
Embodiments of the present inventive concepts relate to a stretch blow moulding machine for stretch blow moulding a plastic preform to form a container, in particular, for carrying out the present method of stretch blow moulding, with at least one blow station comprising a blow mould. The at least on blow station can also comprise a stretching rod element that stretches the plastic preform in a stretching direction.
Generic methods of stretch blow moulding are sufficiently known with respect to the prior art. For example, any type of plastic containers can be produced according to a two-stage method of stretch blow moulding from generally rotationally symmetrical plastic preforms, by expanding these preforms by means of process air in a blow mould of a stretch blow moulding machine to form the plastic containers. Before they are expanded, these plastic preforms are generally heated to a defined process temperature in order to, with a corresponding shaping temperature, facilitate a targeted shaping process during stretch blow moulding.
The shaping temperature is of great significance in the prior art. According to the prior art, with the aid of IR radiation or other radiation sources, the plastic preform is provided with a temperature profile. The temperature profile is usually uniformly distributed about the circumference and it varies as a function of the process merely in the longitudinal direction of the plastic preform. After the plastic preform has been appropriately heated, this is followed by the actual shaping process in which the plastic preform is expanded by means of compressed air in particular in a stretch blow moulding process to form the particular plastic container.
A significant characteristic of a conventional plastic preform may be seen in the fact that it consists of a mouthpiece region already produced to final dimensions and an adjoining body region which is still to be deformed. The material to be processed in this case, generally including polyethylene terephthalate (PET), is also characterized in particular by the fact that during expanding or respectively elongating it tends towards self-hardening, or strain hardening. This effect is traditionally used specifically in the production of such plastic containers in order to optimise the wall thickness distribution of the particular plastic container.
In particular, a warmer material region is stretched and deformed until a stretching resistance resulting from the self-hardening is greater than a stretching resistance of an adjacent colder material region. The expansion of a plastic preform tip region, which is remote from the mouthpiece region, and which substantially forms the later base region of the plastic container, is more problematic since substantially less deformation or respectively expansion of the plastic preform occurs in this manner.
Disadvantageously, only very little material can be stretched out of the centre of the tip region. The resulting negative consequences include in particular a thick material area in this center. Thus, disadvantageously, this thick material area does not contain used material, which necessitates an increased cooling requirement, so that a desired stability of shape can be achieved. In the prior art, a mechanism is provided in the base of the blow mould in order to reduce or respectively to avoid the risks of formation of such a thick material area. However the construction of this mechanism is relatively complex.